Tag: Neutrino

The largest structure ever in the history of mankind is being built for the smallest particle of matter known to mankind. Called the KM3Net Telescope, the structure will be a neutrino detector, taller than the Burj Khalifa, but buried under 3200 meters of water! The whole structure is the latest in the series of larger and larger neutrino detectors and will be the product of a pan-European contribution. It will be the second largest structure in human history, second only to the Great Wall of China.

The neutrino detector (Courtesy: KM3Net)

The super-structure

The super-structure will consist of long cables, holding an optical modulus at the end of each. Each of these optical moduli is a standalone sensing unit, sensitive to light. It will consist of 31 photo-multiplier tubes (or PMTs), which are sensitive light-detectors. All of this will be sealed up as one unit inside a 17-inch glass sphere. The detector will consist of a huge number of such optical moduli!

The optical modulus

Why neutrinos are such a headache!

Neutrinos are notoriously hard to detect. They interact only via the weak force (which is also responsible for the decay of heavy nuclei) with other particles and no other force. They leave no trails in the conventional detector chambers, as they are not charged. They do not affect other matter gravitationally, as they are massless (or have extremely small mass!). In order to detect them, the best strategy is to let it hit a proton and convert it into a neutron. This liberates a positron (or anti-electron). The positron emitted travels at such high speeds that it emits radiation called Cherenkov radiation. (Cherenkov radiation is the radiation emitted by a charged particle when it moves faster then light in that medium. Thus, here the electrons/positrons have to move faster than light moves in water). This radiation is detected by the PMT’s. The radiation comes in the form of a cone. This is how the neutrino is indirectly detected.

The KM3Net project is also a Cherenkov type detector, like the famous T2K detector in Japan, which is the largest right now.

Tall, Taller – Tallest!

The whole structure will be taller than the Burj Khalifa, the tallest building in the world. But, this will not be noticed, because it will be underwater. The chamber will be filled with water, as it has a high density of protons. It is meant to detect neutrinos coming through the Earth, and through the sea-floor. This is possible, since neutrinos interact very weakly.

The most common particle in the Universe is also the most mysterious, but it seems that scientists might have got something correctly predicted about it. Neutrinos have been noticed to disappear’ in the Double Chooz experiment and this is being interpreted as the manifestation of the elusive neutrino oscillation signature. Electron anti-neutrinos have been noticed to simply disappear meaning that they are actually turning into tau anti-neutrinos, which we have no way of detecting. Technically, scientists are measuring the third mixing angle’ or Î¸13.

The Double Chooz experiment

Oscillations of neutrinos

Neutrinos are strange because they do not behave in conventional’ ways. One form of neutrinos can change into another, provided neutrinos have mass, however small it might be. There are three types of neutrinos electron neutrinos, muon neutrinos and tau neutrinos. The names are given according to the particle they accompany in a doublet.

Experimental evidence suggests that one form of neutrinos changes into another and this is through a process called see-saw’ mechanism. In other words, the neutrino exists in a mixed’ state and we detect only one of the constituent states. (If you think this is weird, just know that this is the staple bread-butter of quantum mechanics.) The amount of mixing is given by angles. The electron (type 1) and muon (type 2) type neutrinos mix via the mixing angle Î¸12. The muon (type 2) and tau (type 3) neutrinos mix via the Î¸23 angle. The electron and the tau neutrinos mix via the angle Î¸13, which happens to be out angle of interest. We know that Î¸13 is very small, but we want to know how small it really is. The fact that it is non-zero is, in itself, remarkable.

Neutrino oscillations. The subscripted letters refer to the type of neutrinos.

The value of the mixing angle and the consequence of that

One of the experiments measuring the Î¸13 is the Double Chooz experiment. It just released the first set of results and it gives a definitive value for this third mixing angle. The value, given in terms of sine squared of double the angle, is

sin22Î¸13 = 0.085 + 0.029(stat) + 0.042(syst),

where the last two numbers represent errors and need not concern us too much at the moment.

The T2K experiment. The walls are lined with photo-detectors. The entire chamber will be filled with water when in operation.

What is interesting is the fact that the other giant experiment in the field of neutrinos the T2K experiment also gives similar results.

The value of Î¸13 is not zero and the two results corroborate one another to give a 3-sigma level confidence on that fact. There are neutrino oscillations between the electron type and the tau type.

This is a theoretically significant result for scientists, who are knee-deep with questions about neutrinos and their properties (and, before you ask, the faster-than-light results are the least of the worries). This will put further constraints on the neutrino masses.

After the astonishing result from the OPERA collaboration of detecting neutrinos travelling faster than speed of light, Fermilab wants to double-check the claim. This is an inevitable step in the direction of validating the apparent finding. If Fermilab’s MINOS data doesn’t find anything that replicates the OPERA observations with high enough confidence, then the OPERA result, despite its hype, will become null and void.

The MINOS experiment at Fermilab

Reproducibility

Here’s the reason why, despite the care and beauty of the OPERA experiment, it needs independent corroboration: every scientific result must be reproducible. Fermilab has an advantage over other neutrino research labs in the world since it already has the data sets from the famous MINOS experiment.

Neutrino Oscillations

MINOS was Fermilab’s version of the Super Kamiokande experiment,. Neutrinos come in three flavours or types electron, muon and tau. The curious thing is that neutrinos can oscillate’ or change between these types. An electron neutrino can become a muon neutrino. A theoretical mechanism, known as the see-saw mechanism, explains this, using certain unknown parameters, which need to be supplied experimentally. Super Kamiokande performed experiments in 1998 and confirmed the phenomenon of oscillation and measured the mixing angle’ too. Fermilab repeated this experiment and found consistent results. This was the MINOS experiment, MINOS standing for Main Injector Neutrino Oscillation Search.

MINOS

Well known to scientists in the neutrino field, but virtually unthinkable to the outside world, is that fact that MINOS had actually detected neutrinos moving faster than light. However, these couldn’t survive analysis and presented only a 1.6 to 2 sigma confidence level, below the 3 sigma needed for validation and way below the 5 sigma needed for labeling it as a discovery. MINOS now plans to sift through their data and put it through rigorous analysis. MINOS should take less than 6 months, since the data is already available to them.

It won’t matter if the OPERA experiment isn’t proved wrong. If Fermilab and T2K don’t reproduce the data, OPERA will be up for grabs. Einstein, thou be stillâ€¦ at least for 6 months.

So CERN has stunned us with a result and this one doesn’t even come from the LHC. The premier European high energy research institute has detected neutrinos that seem to move at a speed greater than that of light, violating one of the most sacred pillars of physics Einstein’s Special Relativity. You must have read about it we posted it here. So what about these faster-than-light neutrinos? Why are so many people all excited about them?

In this article, I will try and explain that, touching upon four crucial points. First we need to understand why people are not ready to believe the result in the first place. Next, we’ll understand whether this is believable or not. Is CERN just tricking us or have they put real hard work behind this before publishing it? Next, we shall talk about the implications of this result, if it is proved right. Lastly, we discuss how there can still be flaws and where some glitches might be found in the coming days.

Unlike the popular media, scientists are treading softly on this result. They are not yet ready to say that Einstein was wrong, although that is what it would imply. They are merely reporting facts at this moment, stating the results as got in the experiment. The result is very possibly wrong, but let’s take a closer look.

What on earth are Neutrinos?

The real heroes of this story, Neutrinos are the slipperiest of all known particles. They carry no charge, almost no mass and interact extremely feebly with other matter and that too via the weak interaction. They’re nearly impossible to detect. They leave no tracks in bubble chambers (no charge), don’t interact with each other to form clumps (no strong interactions, like those of protons and neutrons) or speak with normal matter particles. Scientists were forced to assume its existence to solve a puzzle (the beta decay problem), and, even though neutrinos have been detected after that by several detectors, their properties remain largely mysterious. They are giving a headache once more.

Why are people not ready to believe it?

Simply put, it’s Einstein. People are not expecting anything new and now they find this! This is just too unexpected. Why take a result so flagrantly conflicting with all known physical results at face value? Wellâ€¦

Is this result Believable?

As an answer the first of our questions, I would go with a Yes‘. The result is totally believable in the sense that the experiment and analysis seem water-tight at this moment. Scientists of the OPERA collaboration have been looking at the data for three years! They have done everything scientifically possible to discredit their own finding, but have only managed to strengthen it.

Remember, we told you in the particle physics articles, what confidence level means? A confidence level, quoted as some n-sigma, n’ being an integer, refers to the amount of confidence the experimenter has on his/her own results. A 3-sigma result is one which is significant enough to be considered a potential for detection’. This means that the doubts are less than 0.3%. We’re just getting warmed up! For a discovery’ we need a minimum of 5-sigma, which is a confidence level of 99.9999%.

The current results are a 6-sigma, at 99.999999% confidence level, high and above the threshold required to get a discovered’ tag!! This still doesn’t mean that it is true. It just means that the possibility that this is merely a statistical fluctuation is extremely small. They two are very close, but not the same.

Schematic layout of the OPERA experiment.

The real motivation for believing in what CERN has found is the methodology they’ve applied in finding out the results. They had found this result 3 years back, but never jumped the gun in publishing it. They checked and re-checked everything, found crucial error bars and found that this result survives. They added more parameters contributing smaller errors, hoping that they’ll somehow add up and then give the necessary’ error bars. They didn’t.

We’ll just talk about the use of GPS and cesium atomic clocks to measure time and how accurately the distance was measured. Since velocity is simply distance divided by time, we need both parameters accurately.